1. Field of the Invention
The present invention relates to a marking based on chiral nematic (also called cholesteric) liquid crystal polymers. The invention also relates to a method for the modification of chiral nematic liquid-crystal films, in which a chiral nematic liquid-crystal layer is locally or selectively (one or more region(s)) modified with the aid of a modifying agent.
2. Discussion of Background Information
Counterfeiting and market diversion of mass produced goods are facilitated if the products are handled on a lot base rather than on an individual item base. In such case counterfeit or diverted products are easily introduced into the supply chain. Producers and retailers would like to be in a position to distinguish their original products from such counterfeit or diverted (parallel imported or smuggled) products at the level of the individual unit that is sold.
In an attempt to prevent counterfeiting marking is currently used extensively; for example, “optically variable” features which exhibits viewing-angle dependent color, have been proposed in the art as authentication means. Among these are holograms (cf. Rudolf L. van Renesse, “Optical Document Security” 2nd ed., 1998, the entire disclosure whereof is incorporated by reference herein, chapter 10) and optical thin film security devices (idem, chapter 13).
Another type of marking in an attempt to prevent counterfeiting is a cholesteric liquid crystal, which exhibits viewing-angle dependent color. When illuminated with white light the cholesteric liquid crystal structure reflects light of a predetermined color (predetermined wavelength range) which is a function of the employed materials and generally varies with the angle of observation and the device temperature. The precursor material itself is colorless and the observed color (predetermined wavelength range) is only due to a physical reflection effect at the cholesteric helical structure adopted at a given temperature by the liquid crystal material (cf. J. L. Fergason, Molecular Crystals, Vol. 1, pp. 293-307 (1966), the entire disclosure whereof is incorporated by reference herein). In particular, in liquid crystal materials (cholesteric liquid crystal polymers (CLCPs)) the cholesteric helical structure is “frozen” in a predetermined state through polymerization and thus rendered temperature-independent.
The chiral nematic liquid crystal phase is typically composed of nematic mesogenic molecules which comprise a chiral dopant that produces intermolecular forces that favor alignment between molecules at a slight angle to one another. The result thereof is the formation of a structure which can be visualized as a stack of very thin 2-D nematic-like layers with the director in each layer twisted with respect to those above and below. An important characteristic of the chiral nematic liquid crystal phase is the pitch p (see FIG. 1). The pitch p is defined as the (vertical) distance it takes for the director to rotate one full turn in the helix.
A characteristic property of the helical structure of the chiral nematic phase is its ability to selectively reflect light whose wavelength falls within a specific range. When this range overlaps with a portion of the visible spectrum a colored reflection will be perceived by an observer. The center of the range is approximately equal to the pitch multiplied by the average refractive index of the material. One parameter which has an influence on the pitch is the temperature because of the dependence thereon of the gradual change in director orientation between successive layers which modifies the pitch length, resulting in an alteration of the wavelength of reflected light as a function of the temperature. The angle at which the director changes can be made larger, thereby tightening the pitch, by increasing the temperature of the molecules, hence giving them more thermal energy. Similarly, decreasing the temperature of the molecules increases the pitch length of the chiral nematic liquid crystal. Other definitions of liquid crystal polymers and liquid crystal phases can be found in M. Barón, Pure Appl. Chem., 2001, Vol. 73, No. 5, pp. 845-895, the entire disclosure of which is incorporated by reference herein.
In order to enhance the level of security of a chiral liquid crystal polymer film a first idea which could come to mind is to superpose a code in the form of a pattern, indicia or a bar code on the liquid crystal polymer film. However, there is always a risk that a counterfeiter tampers with the code and manually applies it onto the liquid crystal polymer film.
A second possibility to overcome this problem is to insert directly the code inside the liquid crystal polymer film. For example, U.S. Pat. No. 6,207,240, the entire disclosure whereof is incorporated by reference herein, describes an effect coating of a cholesteric liquid crystal polymer (CLCP) with viewing angle dependent reflection color, which further comprises absorption type pigments exhibiting a specific absorption color. A marking, such as a symbol or a text, is generated in the CLCP coating by laser irradiation. The laser radiation carbonizes the CLCP material in the irradiated area. As a result, the color of the substrate on which the CLCP is coated, or the color of absorption pigments incorporated into the CLCP, becomes visible in the irradiated area. However, the method requires high-power lasers to carbonize the material such that the markings are visible.
Another possibility is described in US 2006/0257633A1, the entire disclosure of which is incorporated by reference herein, which is applied not only to liquid crystal polymers but to polymers in general. The method consists of applying a permeating substance to a predetermined region on the surface of the polymer substrate and bringing a supercritical fluid into contact with the surface of the polymer substrate to which the permeating substance has been applied to cause the permeating substance to permeate into the polymer substrate. The method makes it possible to selectively (partially) modify a portion of the surface of the polymer. However, for industrial processes where a high marking speed for a large number of items is required the method is complex and expensive to implement.